Background The focus of this study was to design and pilot a questionnaire to measure young people's attitudes to breastfeeding using the Theory of Planned Behaviour (TPB). This is intended for subsequent use in a large-scale attitude survey, which in turn will inform the design of a breastfeeding intervention programme with adolescents. Methods The first three phases of a research programme employing the theory are described: belief elicitation, questionnaire development and a pilot study. Firstly, an elicitation study to identify the modal salient beliefs underlying young peoples' motivations to breastfeed, using six semi-structured focus groups with 48 young people, was performed. Secondly, the measurement instrument was constructed, incorporating all the key theoretical constructs and both direct and belief-based measures. The questionnaire was then piloted on a sample of 121 female and male schoolchildren to identify and assess the relative importance of the determinants of breastfeeding intention. Results The questionnaire proved to be reliable, and preliminary analysis provided strong support for the predictive power of the TPB. Conclusions Some key issues involved in the operationalization of the theory are highlighted, which may be of interest to researchers involved in the design of TPB questionnaires for use in other intervention programmes.